1. Field of the Invention
The present invention relates to a low floor type motorcycle having a cover member for covering a vehicle body frame, an engine on the lower side of the cover member, and exhaust pipes extending respectively from cylinders of the engine, the exhaust pipes being gathered at a gathering portion.
2. Description of Background Art
A low floor type motorcycle having a cover member for covering a vehicle body frame and an engine on the lower side of the cover member has been known. (Refer, for example, to Japanese Patent Laid-open No. 2002-79982 (FIG. 2).)
In FIG. 2 of Japanese Patent Laid-open No. 2002-79982, the motorcycle 1 includes a vehicle body frame 2, and an engine unit 41 (hereinafter referred to simply as the engine 41) is suspended from the vehicle body frame 2.
The engine 41 is a 2-cylinder engine comprising a balancer shaft 67 (hereinafter referred to as the balance shaft 67) on the upper side of a crankshaft 61 at the mating plane between a cylinder block 47 and a crankcase 48.
The balance shaft 67 provided for the 2-cylinder engine as above-mentioned is generally a primary balance shaft for reducing primary vibration. When the balance shaft 67 is provided for the 2-cylinder engine, the primary vibration of the engine 41 can be reduced.
In this engine form, however, secondary vibration is left as it is, though the primary vibration can be canceled by the balance shaft. As a result, in the low floor type motorcycle having resin-made exterior component parts, the secondary vibration generated at the engine 41 may be transmitted, for example, a cover member or the like constituting a part of the exterior component parts, resulting in the generation of sounds of the cover member or the like.
As a countermeasure against the vibration of the exterior component parts, there is a method in which rubber mounts are adopted in a support structure for the engine so as to insulate the vibration of the engine. In this case, however, the number of component parts is increased, since members for augmenting the rigidity of the vehicle and the like are separately needed for enabling high-speed running.
Meanwhile, for reducing the secondary vibration, an increase in the number of cylinders of the engine is effective. For example, when a 4-cylinder engine is applied to the low floor type motorcycle, the secondary vibration can be reduced. However, application of a 4-cylinder or 6-cylinder engine increases the engine width (the side of the engine in the vehicle width direction, here and hereinafter) too much, thereby making it difficult to retain the form of the low floor type motorcycle as a motor scooter type vehicle.
In addition, a low floor type motorcycle in which exhaust pipes extend respectively from cylinders of an engine and are gathered at a gathering portion has been known. (See, for example, Japanese Patent Laid-open No. Hei 11-99980 (FIG. 2).)
FIG. 2 of Japanese Patent Laid-open No. Hei 11-99980 is a plan view of a motorcycle having a low floor type floor, in which the motorcycle 1 has a horizontally opposed type internal combustion engine 3 (hereinafter referred to as the engine 3), left and right cylinders 4l and 4r possessed by the engine 3 are disposed respectively on the front side of left and right floors 2l and 2r, an exhaust pipe 32r extends rearwards from the cylinder 4r, an exhaust pipe 32l extends rearwards from the cylinder 41, and the exhaust pipe 32l extends to the side of the exhaust pipe 32r and gathered to the exhaust pipe 32r.
However, since the exhaust pipe 32l extending rearwards extends to the side of the exhaust pipe 32r and gathered to the exhaust pipe 32r, it is necessary to secure a sufficient space on the rear side of the engine 3, for example, to secure a sufficient spacing between the exhaust pipe 32l and a rear fork 14 permitted to swing up and down (hereinafter referred to the rear swing arm 14).
In addition, since the exhaust pipes 32l and 32r extend toward the rear side of the engine 3, the position of the gathering portion where the exhaust pipe 32l and the exhaust pipe 32r are gathered together is restricted to the rear side of the engine 3. Therefore, the length of each exhaust pipe from each cylinder of the engine 3 to the gathering portion must assuredly set to be not less than a predetermined value. In addition, in the case where one of the exhaust pipes extend to the side of the other of the exhaust pipes as in the prior art, restrictions may be generated as to the adjustment of power performance between the cylinders; for example, there is generated a large difference in length between the exhaust pipes.
Given the above described problems, there is a need for a technology by which the secondary vibration of an engine can be largely reduced and an increase in the engine width can be suppressed. Further, there is a need for a low floor type motorcycle such that it is unnecessary to provide a space for disposing exhaust pipes on the rear side of the engine, and such that adjustment of power performance between cylinders can be performed in a flexible manner.